1. Field of the Invention
The present invention relates to a radiation image reproducing apparatus and, more particularly, to a radiation image reproducing apparatus of the type which senses a radiation image of an object stored in a stimulable phosphor sheet or like recording medium to record it on another recording medium.
2. Description of the Prior Art
A radiation photographing system has been proposed, which uses a stimulable phosphor sheet as a recording medium, as disclosed in U.S. Pat. No. 3,859,527, for example. The recording medium is exposed to a radiation transmitted through an object to store a radiation image thereof. Afterwards, the recording medium is stimulated by stimulating rays so that the radiation image may be read photoelectrically to be recorded on another recording medium as a visible image of the object.
In an image reproducing apparatus applicable to the system described above, it is a usual practice to subject radiation image data read out to various image processings such as a gradation processing and a spatial frequency processing and, then, to record the processed data on a photo film or like recording medium as a hard copy, or to reproduce it as a visible image on a cathode-ray tube (CRT) or any other suitable display device.
When the stimulable phosphor sheet is exposed to an imagewise radiation of an object, information on the object and exposure is entered into the image reproducing apparatus to be stored in a file. Some of the information is read out of the file in the event of the subsequent reproduction of the object's image to be recorded as visible information on a hard copy of the reproduced image. The visible object and exposure information may be utilized for diagnoses by a doctor, for example. Such information may be typified by identification (ID) data on a patient or like object and exposure conditions which include exposed object's part and exposing method.
In the reproduction of a radiation image, it is generally desired that image processings be carried out to make the reproduced image appear easy to see for a specific application. Basically, the image processings include a gradation processing and a spatial frequency accentuating processing.
Gradation processing includes a video signal processing which controls a relationship between an optical density of a reproduced image on a recording medium and a level of a recording signal to be recorded on the recording medium, as taught in U.S. Pat. Nos. 4,276,473, 4,310,886 and 4,302,672. For example, the curvature of a curve or the gradient of a line approximating the curve as well as the level thereof are made variable to match them to specific properties of an image to be processed. Supposing that an object is the thorax of a human body, the contrast may be lowered in the region of the heart and raised in the region of the lungs in order to greatly improve the diagnostic performance for the lung region without deteriorating that for the heart region. Likewise, the contrast of the backbone may be lowered and that of the heart and lung regions raised for the purpose of facilitating visual analysis of the thorax. The configuration, either concave or convex, and gradient of the previously mentioned optical density to signal level curve, the density level and the like have to be selected out of, for example, ten different types depending upon the diagnostic purpose of a radiation image.
The spatial frequency accentuation or emphasis is an image processing for accentuating a video signal in a specific spatial frequency range when reproducing a radiation image, as described in U.S. Pat. Nos. 4,315,318 and 4,387,428, for example. When a video signal is accentuated in a very low spatial frequency range with high spatial frequency components less accentuated, the resulting image will accompany a minimum of noise components to facilitate interpretation of the image. There are, for example, ten different types of spatial frequency ranges to be accentuated and ten types of accentuation degrees, which have to be properly selected to attain a desired diagnostic purpose.
Thus, to achieve a reproduced image optimum for any diagnostic purpose, it is necessary to select one out of ten different types of gradation processings, one out of ten frequency ranges for spatial frequency accentuation and one out of ten degrees of accentuation, that is, one out of 1,000 different combinations in theory.
In this manner, image processing conditions have to be selected out of numerous combinations. Exposures to radiations are usually conducted by doctors or radiographers who are not always experts in image processing techniques. A system which forces the selection concerned on such persons will not be a system which is easy for a user to operate.
A plurality of stimulable phosphor sheets (recording layers) may be laid one upon another and exposed to radiation at the same time, as is sometimes the case with multi-layer tomography. In such a structure, the amount of absorbed radiation energy progressively decreases from the top layer toward the bottom layer, degrading the image quality and thereby the contrast accordingly. This has to be compensated for by modifying ordinary image processings to a greater degree sequentially toward the bottom layer or sheet. In practice, however, it is difficult for one who lacks full knowledge of image processings to select appropriate conditions for the modification.